Heat treatment of metals



Patented May 14, 1940 UNITED STATES PATENT OFFICE 2,201,111 HEATTREATRIENT 0F METALS Josef Martin Michel, Bitterfeld, Germany, assignorto I. G. Farbenindustrie Aktlengesellschaft, Frankfort-on-the-Main,Germany No Drawing. Application August 30, 1939, Serial No. 292,607. InGermany September 2, 1938 5 Claims.

This invention relates to improvements in the heat treatment of metals,especially of aluminium base alloys.

When improving aluminium base alloys byheating in a salt bath followedby quenching in water, it hasvbeen observed that the workpieces,

particularly sheets, treated in this manner ex-.

hibit local stains or discolorations. In order to avoid such staining ordiscoloration it has been suggested to add chromates to the bathof fusedsaltpetre employed for the heating, in the assumption'that the stainingis due to a corrosion of the workpieces caused by the decompositionproducts of the saltpetre which are produced in the course of use of theheating bath over a long period. Although this leads to a substantiallessening of the staining, the latter to some extent still persists.

It has now been found that the staining is in part also due to an actionof the quenching bath upon the metal. When workpieces, forexamplesheets, with a completely clean surface are quenched in pure water, nostaining takes place; in practical operation, however, appreciablequantitles of the fused salts constituting the bath in whichthe metal isheated prior to quenching adhere to the sheets and become dissolved inthe quenching water when the sheets are placed therein. As successivebatches of work-pieces are heat-treated'and quenched, the quenchingwater thus takes up progressively increasing quantities of the latter incourse of time and becomes correspondingly more alkaline. It has beenobserved that this alkalization of the quenching 'water occurs not onlywhen the fused salts employed in the heating bath are itself weaklyalkaline, but also when any" alkalinity of the fused salts em ployed asthe heating bath has been compensated by the addition, to the heatingbath, of salts having an acid reaction, for example dichromates, untilthe reaction was neutral. This latter phenomenon is possibly due to adecomposition, occurring on contact with water vapor, of the saltpetre(which is generally the main constituent of the heating bath),accompanied by the formation of oxides of nitrogen and freealkali. Thedegree of staining of the metal has been found to be directly dependenton the alkalinity of the quenching water and to increase with increasingalkalinity of the latter. o

Based on the foregoing observations, the invention provides a processfor the heat treatment of metals, especially aluminium base alloys, by'5 heating in a fused salt bath containing a substantial proportion ofsaltpetre and subsequently quenching in an aqueous medium, in which thepH-value of the quenching medium is preferably by the addition ofsuitable quantities of acids kept constantly numerically below 7 andpreferably at about 6.

Acids having such, anions as are also present, in the form of theirsalts, in the fused salt bath employed for the heating are preferablyused for this purpose. Thus, for example, when using heating bathsconsisting of saltpetre with an addition of chromates, the acidificationof the quenching bath can be effected with the aid of chromic acid,while it is found convenient to regulate the addition of acid in such amanner that the pH-va lue of the quenching water is numerically closelybelow 7, i. e. in such a manner that a certain proportion ofmonochromate remains present in the quenching water together with thedichromate.

Since the amount of salts dissolving in the quenching water in thecourse of time is not inconsiderable and, when the salts used in theheating bath are expensive,'represents a source of appreciable loss, itmay sometimes be advantageous to recover the salts from the quenchingwater. To this end, in order to ensure a sumcient concentration of thesesalts in the quenching bath, the volume of the latter is kept asconstant as possible, the evaporation losses merely being made up withfresh water as required; the resulting progressive enrichment of thequenching water with dissolved, salts has no adverse effect on thequenching efiiciency thereof. By suitably selecting and proportioningthe acids to be added to the quenching water for the purpose ofadjusting the pH-value thereof in accordance with the invention, it isthen possible to make the concentration ratio of the salts dissolved inthe quenching bath approximately correspond to that prevailing inthesalt mixture to be supplied to the heating bath; to this end it isnaturally necessary that the latter mixture should also have a neutralor weakly acid reaction. After having attained a sufficientconcentration, the quenching solution is then evaporated to dryness,whereby a saltmixture is recovered which, by virtue of its composition,is directly utilizable for making up the heating baths.

Ezampla -For soaking-sheets of an aluminium alloy at a temperature of500 C., a fused salt bath is used which is composed of:

r Per cent NaNOa 65 KNO: 30 moi The quenching'bath, which has a capacityoi. 20 cubic metres and is initially charged only with fresh water,contains, after treatment of 18,000

square meters of sheets Grams/litre Na+ 61.6

K+ 34.8 NOa- 201.2 Nor 6.7 CrOr- 10.7

and has a pH-value of 8.2. In order to adjust the pH-value to a value inthe proximity of 6.75 kgs. of nitric acid (40 per cent HNOa) or kgs. ofchromatic anhydride are added to-the bath, whereby at the same time theconcentration ratio of the salts contained in the quenching water ismade to correspond approximately with the relative portions of the saltsconstituting the soaking bath, so that after evaporation of the watercontent of the quenching bath, the salt recovered (6300 kgs.) can beused directly for making up 1. An improvement in the heat treatment ofmetals, especially of aluminium base alloys, by a process involvingheating successive batches of metal in a fused salt bath substantiallyconsisting of saltpetre and subsequently quenching such batches in anaqueous medium, which improvement consists in constantly maintaining thepH- value of the quenching medium numerically below about 7. v l

2. An improvement in the heat treatment of metals, especially ofaluminium base alloys, by a process involving heating successive batchesof metal in a fused salt bath substantially consisting of saltpetre andsubsequently quenching such batches in an aqueous medium, whichimprovement consists in constantly maintaining the-pH- value of thequenching medium numerically at about 7.

,3. An improvement in the heat treatment of metals, especially ofaluminium base alloys, by a process involving heating successive batchesof metal in a fused salt bath substantially consisting of saltpetre andsubsequently quenching such batches in an aqueous medium, whichimprovement consists in constantly maintaining the pH- value of thequenching medium numerically below 7 by the addition of acids. v

4. An improvement in the heat treatment of metals, especially ofaluminium base alloys, by a process involving heating successive batchesof metal in a fused salt bath substantially consisting of saltpetre andsubsequently quenching such batches in an aqueous medium, whichimprovement consists in constantly maintaining the pH- value of thequenching medium numerically below about '7 by the addition of acids,having such anions as are alsopresent, in the form of their salts in thefused heating bath, recovering the salts dissolved in the quenching bathfrom time to time, and supplying such recovered salts to.

value of the quenching medium numerically below about. 7 by the additionof acids, having such anions as are also present, in the form of theirsalts in the'fused heating bath, recovering the salts dissolved insthequenching bath from time to time, and supplying such recovered salts tothe heating bath, thev acids to be added to the quenching medium beingso selected and proportioned that the concentration "ratio of the saltsdissolved in the quenching medium is made to correspond approximately tothat prevailing in the salt, mixture constituting the heating bath.

JOSEF MARTIN MICHEL. 6

